Devices are known for measuring the shape or position of an object that comprise a stand supporting a series of hinged-together measurement segments and angular position sensors for sensing the angular positions of the measurement segments. To manipulate the measurement apparatus, it is necessary to support the measurement segments so as to bring the end of the last measurement segment into contact with a point whose position is to be determined, either relative to a reference surface, or else relative to some other point against which the end of the last measurement segment is brought subsequently, with the relative position of these points being computed by a processor unit as a function of the information given by the angular position sensors when contact is being made with the end of the last measurement segment.
At present, the measurement segments are supported either manually or else by means of motors associated with the measurement segments. Supporting the measurement segments manually tires the user and limits the capacity of the user to manipulate the apparatus. In particular, when the measurement segments are of large dimensions, e.g. for measuring the positions of points that are a long way apart, it can become impossible for the measurement apparatus to be manipulated by a single user.
To avoid this drawback, proposals have been made to motorize the measurement segments, i.e. to place not only angular position sensors at the hinges, but also to place assistance members such as drive motors for driving the measurement segments relative to one another, or more simply to provide springs at the hinges between the measurement segments or to provide cables associated with the measurement segments and connected to counterweights to balance the gravity forces to which the measurement segments are subjected. Documents GB-A-1 498 009 and EP-A-0 237 698 illustrate such solutions. Regardless of the assistance means used, the hinges between the measurement segments are subjected to a reaction force which opposes the force of gravity to which the measurements segments are subjected. These gravity and reaction forces can give rise to hinge wear, or merely to deformation, in particular because of segment bending, thereby disturbing the measurement performed by the position sensors such that these apparatuses cannot be used for very high accuracy measurements, e.g. measurements where it is desired to determine the position of a point to within one micron.
The state of the art also includes hinged-arm robot structures (see, for example, Documents EP-A-0 078 113 and DE-A-3 730 873). Finally, Document U.S. Pat. No. 4,343,092 illustrates a universal joint mount for a measurement apparatus, with the apparatus being supported thereby, thereby increasing the forces at the hinges.
An object of the present invention is to provide very high accuracy measurement apparatus capable of being used without tiring a user.